It is well known in the art that the rate of fuel flow from a variable venturi carburetor fuel bowl, for any given size of fuel metering restriction employed, is dependent upon the pressure differential existing across the fuel within the bowl as measured, for example, from above the level of the fuel within the bowl to the discharge orifice of the fuel metering system.
Theoretically, in order to achieve a proper fuel-air mixture discharged by the carburetor, the relationship sought in one where a particular mass of fuel (such as pounds of fuel per hour) is metered as to be mixed with a particular mass of air (also such as pounds per hour).
In doing this, carburetors employ either a fixed or variable venturi, or some functionally equivalent structure, within the induction passage so that as air flows therethrough a reduction in the pressure (often referred to as venturi vecuum) of the air is achieved in the vicinity of the venturi throat. The value of the venturi vacuum is of a variable magnitude generally indicative of the rate of flow of such air through the venturi.
However, the flow of air through such venturi does not necessarily indicate that the mass rate of flow of such air is constant even though the venturi vacuum generated thereby should remain constant. That is, the generated venturi vacuum is produced in response to the volume rate of air flow and not the mass rate of air flow. Consequently, as either barometric pressure changes due to atmospheric conditions or ambient pressures change due to changes in altitude, the volume rate of air flow many in fact remain constant but such a volume rate of air flow will represent differing values of mass rate of air flow as between two different atmospheric pressures. For example, while driving through the mountains a volume rate of flow of air developing the same venturi vacuum as previously experienced at, for example, sea level, will actually represent a lesser mass rate of flow of air because of the decrease in density of the air in such higher altitudes. However, since the rate of metered fuel flow is primarily dependent on the vacuum generated at the venturi (and consequent pressure differential across the fuel in the fuel bowl) substantially the same rate of fuel flow is metered to the less dense air of the higher altitude as was metered to the more dense air of the lower altitude.
Obviously, at times, the above results in a somewhat overrich (in terms of fuel) fuel-air mixture delivered to the engine, which, in turn, to that degree increases engine exhaust emissions.
The above problems have been known in the prior art and heretofore attempts have been made to compensate fuel metering requirements in respect to changes in barometric pressure or altitude. Such prior art attempts have taken the form of, for example, a bellows employed for varying the effective area of a restriction used to meter the fuel flow. The determination of such effective area was accomplished as by a variably positioned metering rod cooperating with a fixed orifice. However, such arrangements have not been accepted for various reasons among which are the difficulty of manufacturing the metering rod to the extremely close tolerances required, the accurate and positive location of the metering rod with respect to the surface defining the cooperating fixed orifice and maintaining such relationship during movement of the metering rod, the rather limited travel of the metering rod in which the entire spectrum of compensation must be achieved, and the propensity for such systems to become impaired by virtue of particles of dirt especially when the effective area of the metering orifice is reduced.
Accordingly, the invention as herein disclosed and claimed is primarily directed to the solution of the above as well as other attendant problems.